In the gas phase, traditional Ziegler-Natta catalysts tend to produce ethylene copolymers having relatively broad molecular weight distributions as well as broad and uneven comonomer distribution. Typically, an uneven comonomer distribution is demonstrated using analytical techniques which show that as the molecular weight of a polymer chain increases, the amount of side chain branching present in the chain decreases. Single site catalysts, on the other hand can produce ethylene copolymers having narrower molecular weights and a more even comonomer distribution among polymer chains of varying length.
It has been shown that the use of trimethylaluminum rather than triethylaluminum as a cocatalyst in combination with a titanium based Ziegler-Natta catalyst can produce ethylene copolymers with reduced values of melt flow ratio (see for example U.S. Pat. Nos. 4,888,318; 5,055,533; and Re 33,683).
Changes in the formulation of Ziegler-Natta catalysts, such as the nature of an internal or external electron donor molecule has allowed for the production of ethylene copolymers with good bulk density and improved performance in film applications, such as improved tear strength, dart impact strength, and optical properties (see for example U.S. Pat. Nos. 5,139,986; 7,893,180; 6,191,239; and 6,228,792).
In some cases, an external electron donor can alter the melting point of an ethylene/1-hexene copolymer made with a Ziegler-Natta catalyst (see U.S. Pat. No. 6,417,301).
Other manipulations of an internal electron donor can cause some Ziegler-Natta catalysts to have a different response to the presence of hydrogen as disclosed in U.S. Pat. No. 7,671,149.
Changes in the amount of co-catalyst fed to a reactor along with a Ziegler-Natta catalyst can also lead to ethylene/1-hexene copolymers having improved dart impact properties and reduced hexane extractables (see U.S. Pat. No. 6,825,293).
Differences in the order of addition of the various Ziegler-Natta catalyst components during synthesis can have a positive impact on the resulting polyethylene copolymer properties as discussed in U.S. Pat. No. 7,211,535.
Various iterations of the Ziegler-Natta catalyst have led to ethylene copolymer compositions having not only reduced melt flow ratios, but also to compositions having a more even comonomer distribution. For example, in U.S. Pat. Nos. 7,651,969, 8,993,693, and 9,487,608, a titanium based Zielger-Natta catalyst having an internal 2,6-lutidine electron donor molecule provides ethylene/1-hexene copolymers having a relatively narrow molecular weight distribution and a “single site catalyst” like comonomer distribution. These resins exhibit a good balance of tear and impact properties when made into film.
The present disclosure provides ethylene copolymers having intermediate molecular weight distributions and intermediate comonomer distributions relative to resins made with traditional Ziegler-Natta catalysts and single site catalysts. The resins show advantages associated with products which arise from both of these catalyst types.